The present invention relates to MOSFET-gated bipolar transistors and thyristors having both turn-on and turn-off capabilities.
Hybrid forms of semiconductor devices have previously been proposed, and some have been commercially produced, which combine, in a single integrated device, a MOSFET input stage with junction bipolar device output stage. Such hybrid devices beneficially combine the high input impedance characteristic of MOSFET devices with power-handling capabilities of bipolar devices.
The term "MOSFET", although an acronym for "Metal-Oxide-Semiconductor Field-Effect Transistor", is employed herein in a broader sense to include various equivalent forms of devices, well known to those skilled in the art. For example, the conductive gate electrode may actually be metal such as aluminum, as is shown in the drawings figures herein, but may as well comprise any suitably-conductive material such as highly-doped polycrystalline silicon of either N+ or P+ conductivity type. Similarly, the gate insulating layer may actually be an oxide, such as silicon dioxide, but may as well comprise another insulating material, such as silicon nitride, or even an oxide/nitride sandwich.
Particularly relevant in the context of the present invention is a MOSFET-gated bipolar transistor, such as is for example disclosed in a paper by Nathan Zommer entitled "Monolithic H.V. BIP MOS", 1981 IEEE International Electron Devices Meeting, pp. 263-265, Dec. 7-9, 1981. As there disclosed, this previous MOSFET-gated bipolar transistor device combines, in a single integrated device, the collector, base and emitter electrode regions of a bipolar transistor with an enhancement-mode MOSFET having its drain and source electrodes connected between the collector and base of the bipolar transistor. When an appropriate gate input signal is applied to the MOSFET, its channel conducts, in turn biasing the bipolar transistor into conduction. However, the integrated structure of this particular prior art device does not include any reliable means for turning off the bipolar transistor, necessitating external devices to provide the turn-off function.
Another form of prior art device particularly relevant in the context of the present invention is a hybrid form of thyristor suggested in commonly-assigned Temple U.S. patent application Ser. No. 331,049, filed Dec. 16, 1981 entitled "SEMICONDUCTOR DEVICE WITH IMPROVED TURN-OFF CAPABILITY". This particular hybrid device includes a four-layer thyristor structure combined with a pair of MOSFET devices in the same integrated structure. Specifically disclosed is a P-channel enhancement mode MOSFET appropriately combined in the integrated structure with the thyristor portion of the device to, when conducting, form a short between the gate and cathode electrode regions of the thyristor device to turn it off. Suggested is an N-channel enhancement mode MOSFET appropriately arranged to bias the thyristor portion of the integrated device into conduction when the N-channel enhancement mode MOSFET is biased into conduction. The gates of the two MOSFETs can be electrically connected together to provide a single gate input terminal. The effect of the single gate input terminal is to turn on the thyristor device when a positive gate voltage is applied, and turn off the thyristor device when a negative gate voltage is applied.
Relevant also in this context is a discussion by B. Jayant Baliga entitled "MOS Gated Thyristor" in Silicon Integrated Circuits--Part B, edited by Dawon Kahng, pp. 265-267, Academic Press (1981). This Baliga article discloses an integrated device structure including a VMOS device for triggering a four-layer thyristor into conduction, and points out that both enhancement and depletion mode devices can be fabricated.
While the above-summarized Temple device is potentially quite useful, its requirement for opposite-polarity gate input voltages for turn on and turn off is somewhat disadvantageous when the device is incorporated in actual circuits. In particular, the requirement for a positive gate voltage to bias the N-channel enhancement mode MOSFET into conduction for turning on the thyristor device presents no inconvenience. However, it is less convenient, in many actual circuits, to provide a negative gate voltage to bias the P-channel enhancement mode MOSFET into conduction for turning off the thyristor device.
By the present invention there are provided three-terminal MOSFET-gated bipolar transistor and thyristor devices characterized by having both turn-on and turn-off capability with a single-polarity input signal which, in the preferred embodiments, is of the same polarity as operating supply voltage to the device when connected in circuit.